Powdered materials are sometimes added with other ingredients to form cable insulation. These powdered materials are sometimes added to cable insulation as an extender or a flame retardant. For example, kaolin is added as extender, while aluminum trihydrate is added as a flame retardant. Also, in the manufacture of cable insulation, a powdered material, such as powdered kaolin, is often added for smoother extrusion, to reduce the cost of manufacturing, to provide deformation resistance to the cable insulation, and to improve “green strength.” Green strength is the ability of an uncured material to resist distortion.
However, manufacturing problems arise from the addition of one or more powdered materials. In particular, the powdered material often does not flow smoothly. Also, other ingredients are often added to such powdered materials, but those additional ingredients are not uniformly distributed throughout the powdered material.
The addition of one or more powdered materials can damage or destroy manufacturing machinery. For example, kaolin is sometimes added as an ingredient in the manufacture of cable insulation, but commercially available kaolin is typically provided as a dry powder. The powdered material is then dropped into a hopper that has an opening at its bottom through which the powdered material flows to a compounding machine. However, such a powdered material does not always flow smoothly. Some powdered materials, such as bottom through which the powdered material flows to a compounding machine. However, such a powdered material does not always flow smoothly. Some powdered materials, such as powdered kaolin, are hydrophilic and can absorb moisture which causes it to clump or cake. Also, flow problems, such as bridging and air pocket formation, can also occur. Bridging, sometimes referred to as arching, occurs when the opening at the base of the hopper is too small or the powdered material is compressed enough to form an arched cap above the opening. Sometime, these flow problems can lead to costly repair or replacement of manufacturing machinery.
One technique to improve the flow of a powdered material is to change the size of the particles making up the powdered material, that is change the particle size distribution of the powdered material. By changing the particle size distribution, the powdered material can be made to flow more smoothly. However, to change the particle size distribution, costly classification is required. Classification classifies each particle according to its particular size. Also, for some applications, changing particle size distribution is undesirable.
Another technique for improving the flow of a powdered material is to treat the material chemically so that it flows more smoothly. By adding the appropriate chemicals, the powdered material can be made to flow more smoothly, but the cost of chemically treating a powdered material is expensive. Furthermore, the powdered material itself is often chemically treated to optimize it for a particular application, and altering the chemical treatment of the powdered material may make it unsuitable for that particular purpose.
In addition to adding chemicals to improve the flow of a powdered material, other ingredients are added to the powdered material to improve its suitability in the manufacture of cable insulation. However, one or more of these additional ingredients are not uniformly distributed throughout the powdered material. One such ingredient is a silane compound which is a hydrophobic chemical compound. For example, calcined kaolin clay requires treatment with silane to prevent moisture absorption during wet electrical testing of medium voltage ethylene-propylene rubber (EPR) insulation. The absorption of moisture during accelerated testing can lead to the formation of trees. However, treating a powdered material, such as kaolin, with silane is approximately half of the commercial cost of kaolin. Furthermore, to form suitable cable insulation, fillers with kaolin must be able to combine chemically with polymers. To combine a filler chemically with polymers, the filler is generally pre-treated with silane, which acts as a coupling agent for the filler and polymers. Preferably, the filler reacts with silane before or during mixing with other ingredients.
Thus, there is a need for improving the flow of powdered materials and to improve the distribution of additional ingredients throughout the powdered material. In particular, there is a need for improving the flow of a powdered material that does not change the particle size distribution, that does not require changing the optimum chemical treatment of the powdered material, and that costs less than presently known methods. Also, there is a need for uniformly distributing other ingredients, such as silane, throughout a powdered material.